Send this to your users;

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

I've called the big guys to help you. Good luck.

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

You may have zoomed web page(s) by accident. Reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl/Command+0 (zero))

• http://kb.mozillazine.org/Zoom_text_of_web_pages

If you have made changes to Advanced font size settings like increasing the minimum/default font size then try the Default font size setting "16" and the Minimum font size setting "none" in case the current setting is causing problems.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)

Make sure that you allow pages to choose their own fonts.

• Tools > Options > Content : Fonts & Colors > Advanced: [X] "Allow pages to choose their own fonts, instead of my selections above"

It is better not to increase the minimum font size, but to use an extension to set the default page zoom to prevent issues with text not being displayed properly.

In Firefox 40, unstyled text inputs use the Segoe UI font on my Windows 7.

You can override this using

input {font-family:sans-serif;}

which would change them to Arial or Helvetica on most systems.

This might be related to theming tweaks for Windows 10, I haven't researched it.

Edit: also select controls, so:

input, select {font-family:sans-serif;}